This invention relates to magnetic tape transports, sometimes referred to as tape units, tape drives, tape decks, etc., for use with a replaceable tape assembly such as that known as a tape cassette or cartridge, and more particularly to digital magnetic tape transports used as a subsystem to enable a host system to obtain access to data on the magnetic tape. Still more particularly, the invention pertains, in such tape a transports, to a method of, and means for, starting reel-to-reel tape travel in either direction against the possibility of tape jamming.
Cassette tape transports, particularly to those operating in streaming mode, as contrasted with start/stop mode, have come to find extensive use as peripherals of computer systems. U.S. Pat. No. 4,163,532, filed by Sakai and assigned to the assignee of the instant application, discloses one such streaming cassette tape transport, or streamer. This prior art streamer comprises a pair of reel motors, to be drivingly coupled one to each reel of a tape cassette, for bidirectionally transporting the tape between the two reels, under the direction of a motor control circuit forming a part of a tape speed control servo. Also included in the tape speed control servo is a tape speed sensor comprising a roll for frictional engagement with the tape, and an encoder for generating a series of pulses representative of the actual tape speed. The motor control circuit causes one of the reel motors to be driven at constant speed in response to the speed sensor output pulses for tape transportation in each direction.
The tape must of course travel under proper, constant tension between the reels in order to enable the transducer to correctly write or read data on the tape. To this end the motor control circuit additionally comprises tension control means which afford constant tape tension in the face of varying tape diameters on both reels.
The streaming tape transport of the noted prior art construction has proved to possess a weakness in conjunction with the behavior of the tape cassette under specific temperature conditions, as has proved by testing. Assume that the tape cassette is put to use at elevated temperatures and then at reduced temperatures. Unavoidably, the reel hubs thermally expand during cassette operation at elevated temperatures and contract at reduced temperatures. The magnetic tape also undergoes such thermal expansion and contraction, but the radial expansion and contraction of the tape rolls are so much less than those of the reel hubs that gaps are created therebetween when the cassette cools after having been used at high temperatures.
Consequently, when tape transportation is started at lower temperatures, the takeup reel will immediately begin rotation with the takeup reel motor, but the tape roll on the takeup reel will not. The complete tape roll on the takeup reel will start rotation only after the gap has been removed by reel rotation. Being practically unloaded, the takeup reel motor will rotate at high speed until the removal of the gap. Thereupon the tape roll on the takeup reel will abruptly start rotation together with the file reel and with the tape roll thereon if there is no tape slack between the two reels, resulting in the sudden loading of the takeup reel motor. If there is a tape slack between the reels, on the other hand, then the takeup reel motor will again be suddenly loaded upon removal of the slack.
Such tape travel at abruptly changing speeds, and such instantaneous changes in load on the reel motors, are hazardous to the proper functioning of the tape speed control servo. The hazard becomes even greater by the presence of the gaps between the reels and the tape rolls thereon and of a tape slack between the reels, because then the traveling speed of the tape does not agree with the rotational speed of the takeup side reel motor. U.S. Pat. Appl. Ser. No. 979,639, filed Nov. 20, 1992 by Fujisawa et al. and assigned to the assignee of the instant application teaches how to take up a tape slack but is silent on the subject of how to remove the gaps between the reels and the tape rolls thereon.
Assume for example that the tape speed is unduly low. The tape speed sensor will produce pulses at correspondingly long spacings. The tape speed control servo will respond to such unduly long tape speed pulse spacings by correspondingly accelerating the takeup side reel motor, which actually may have been in rotation at high speed. The tape will then start traveling at inconveniently high speed and thus have to be decelerated greatly. The most likely result has been the hunting of the tape speed control servo. Such hunting has led to the jamming of the tape between the reels and the cassette housing in the worst case.